Children are not adults. It is irrelevant that adults socialise with all ages. Children have different needs.
No, children are not adults. But there is absolutely nothing wrong with a child learning to socialise with adults and children of other ages as well as children of their own age. I'm not sure why you are having difficulty with this concept. It's not "either/or." It's "in addition to."
I am assuming that the socialisation many severely disabled kids may get in mainstream schools would not be great for developing social skills.
Well, see there's where you've gone so incredibly wrong. You are assuming and making a blanket statement and thereby "othering" children with disabilities, rather than looking at them as individuals with varying needs and abilities.
Again, think about that.
Equals means where there are not differential power relationships. It is not about perception, but about fact. An adult and older kid will not treat a younger child the same as another younger child.
Even relationships with "peers" having variances in power. Cliques, disabilities, perceived social class differences, gender, race, religion, background. As children age, these things seep into their socialisation.
Again, I don't understand why you are having difficulty understanding that most home educated children do generally socialise with some children their own age. The difference is that it's not exclusively with children their own age, so they are also exposed to children in other age groups as well as adults.